1. Field of the Invention
The invention relates in general to an N-well or a P-well at a floating stage. More particularly, the invention relates to a method of protecting an N-well or P-well at a floating stage from being damaged by plasma without degrading electrical characteristics or performance.
2. Description of the Related Art
In an integrated circuit, after the formation of certain devices, there are still some processes to be performed to complete the circuit layout. For example, after a PMOS or an NMOS is formed on a substrate, to obtain an electrical connection between PMOS or NMOS and other devices or terminals, a conductive layer is formed and patterned. While patterning the conductive layer, an etching step is inevitable. The plasma or other charged particles used to etch the conductive layer very often damage the NMOS or PMOS formed on the substrate.
Similarly, for a substrate comprising a floating well having a conductivity type different from that of the substrate, that is, the well at a floating stage without electrically connecting to other component or device, the above mentioned plasma damage is also very likely to happen.
Considering a P type substrate comprises an N-well therein. The P type substrate is typically placed on or connected to a chuck which is grounded. Therefore, any charged carriers coming from the plasma can be directed to the chuck to be grounded via the P type substrate. However, if the charged carriers are absorbed by the N-well, these carriers have no path to dissipate or to be neutralized. As a result, the charged carriers accumulate in the N-well, and thus, the electrical characteristics of the N-well or even the quality of gate oxide formed thereon is seriously degraded.
In addition, since the N-well is formed in and adjacent to the P type substrate, plus that the P type substrate is grounded, a potential difference is caused between the P-type substrate and the N-well. Therefore, if a protection structure across the N-well and the P type substrate is formed to resolve the problems caused by plasma damage, with this potential difference, the performance and electrical characteristics will be degraded.